Recombinant plasmids for the expression of polymo virus middle tumor antigen (MT) were constructed in order to obtain a large quantity of the antigens and to study their properties. The plasmids contain an ampicillin resistance gene, lambda phage PL promoter, CII, and a portion of the 0-gene that was fused with MTcDNA. The expression of the hybrid gene (OMT) was controlled in temperature shift experiments. The host E. coli (CI857) strain carries a temperature sensitive mutation in the CI gene that controls PL promoter. The OMT produced in the E. coli was immunoprecipitated specifically by MT monoclonal antibody. The OMT was poorly auto- or trans-phosphorylated by comparison to authentic MT. Antibody specific for MT was produced by injection of OMT into rabbits. The experimental system established may be applied to produce oncogenic proteins of other viral and cellular origins. The mechaisms controlling oncogene expression are studied by cloning and transfection of the hybrid fused DNA consisting of the 5 feet control regions of the human metallothionein (Met) gene and oncogenes cloned from viruses and normal or tumor cell lines. These studies will give insight to the molecular mechanisms of gene expression associated with the oncogenic processes.